$ F = \left[\begin{array}{rr}3 & 4 \\ 0 & 5\end{array}\right]$ $ E = \left[\begin{array}{rr}-2 & -2 \\ -1 & 4\end{array}\right]$ What is $ F E$ ?
Explanation: Because $ F$ has dimensions $(2\times2)$ and $ E$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ F E = \left[\begin{array}{rr}{3} & {4} \\ {0} & {5}\end{array}\right] \left[\begin{array}{rr}{-2} & \color{#DF0030}{-2} \\ {-1} & \color{#DF0030}{4}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ F$ , with the corresponding elements in column $j$ of the second matrix, $ E$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ F$ with the first element in ${\text{column }1}$ of $ E$ , then multiply the second element in ${\text{row }1}$ of $ F$ with the second element in ${\text{column }1}$ of $ E$ , and so on. Add the products together. $ \left[\begin{array}{rr}{3}\cdot{-2}+{4}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ F$ with the corresponding elements in ${\text{column }1}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{-2}+{4}\cdot{-1} & ? \\ {0}\cdot{-2}+{5}\cdot{-1} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ F$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{-2}+{4}\cdot{-1} & {3}\cdot\color{#DF0030}{-2}+{4}\cdot\color{#DF0030}{4} \\ {0}\cdot{-2}+{5}\cdot{-1} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{3}\cdot{-2}+{4}\cdot{-1} & {3}\cdot\color{#DF0030}{-2}+{4}\cdot\color{#DF0030}{4} \\ {0}\cdot{-2}+{5}\cdot{-1} & {0}\cdot\color{#DF0030}{-2}+{5}\cdot\color{#DF0030}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}-10 & 10 \\ -5 & 20\end{array}\right] $